1. Field of the Invention
The present invention relates to a process for dewatering sludges, especially for dewatering sewage sludges by the formation of flocs caused by addition of flocculant to the sludges.
2. Description of the Prior Art
There has been known a process for dewatering sludges in which a sludge such as surplus activated sludge, sludge from alum coagulation, and so on, is first treated by adding thereto a first high molecular weight flocculant of cationic or anionic nature having an electric charge of polarity opposite to that of the sludge while agitating the sludge and then, adding thereto a second high molecular weight flocculant of anionic or cationic nature having an electric charge of polarity opposite to that of said first flocculant. The thus-formed flocs are then subjected to a dewatering operation.
This prior process thus consists in first forming a primary floc by neutralizing the electric charge of the sludge by addition of a first high molecular weight flocculant, and then causing formation of giant flocs by addition of a second high molecular weight flocculant, before carrying out the dewatering operation. This process attains a better dewatering performance due to the formation of giant stout flocs, even for a sludge that will merely form small and feeble flocs upon addition of either an anionic or cationic high molecular weight flocculant only. This prior process exhibits, however, narrow permissible ranges for the amounts of addition of both the cationic and anionic high molecular weight flocculants, in addition to a narrow permissible range of the dosage ratio between both flocculants, so that regulation and control in practical applications where the variations in the concentration and properties of the sludge are considerable, are substantially difficult. Moreover, the process shows a further shortcoming in that, although the flocs formed are large and stout, they form a tenacious dewatered cake and in addition, the moisture content of the dewatered cake from this process is high as compared with that of a cake obtained by the sole addition of a cationic or anionic flocculant, so that the filtration velocity is decreased.